I Samuel 28:1-2

Now it happened in those days that the Philistines gathered their armies together for war, to fight with Israel. And Achish said to David, “You assuredly know that you will go out with me to battle, you and your men.” So David said to Achish, “Surely you know what your servant can do.” And Achish said to David, “Therefore I will make you one of my chief guardians forever.”